The present invention relates to a method and an apparatus for removal of filtrate from thickeners, filters, washers and the like devices. The method and the apparatus according to the invention are particularly suitable in dewatering devices of the wood processing industry i.e. in pulp and paper mills.
In pulp and paper manufacturing many devices of different types have been used for separating liquid (mainly water) from a fiber suspension. In washing or thickening processes, the fiber suspension is fed onto a wire or similar liquid-pervious, planar or curved members. A lower pressure (vacuum) prevails at the opposite, downstream surface of the member. The resulting pressure difference removes the liquid. The reason for separating liquid is mainly to increase the consistency of the suspension or to remove from the suspension liquid which contains chemicals or other liquids. The removal of liquid is always based on the difference of pressure prevailing on the opposite surfaces of the fiber layer due to which the liquid moves towards the lower pressure. One known method of creating a pressure difference is to pressurize the whole system. This is called pressurized thickening and the devices used are in principle pressure vessels. Another, more conventional, method is the so-called vacuum thickening in which a vacuum is generated on one side of the fiber layer which draws liquid from the fiber layer.
The vacuum is usually generated by a drop leg. The drop leg is typically about 10 meters long. This means that the thickening device or the washer in which the fiber layer to be drained must be located approx. ten meters higher than the filtrate tank which communicates with the filtrate space of the thickener or the washer via a pipe in such a way that air is prevented from flowing back to the thickener or the washer. The downwards flowing column of a liquid and gas mixture separated from the fiber layer causes suction at the fiber layer. The system is closed relative to the drop leg pipe and the liquid cannot flow from the pipe to the filtrate tank as the fiber layer in the thickener or the washer forms a "sealing" in the top end of the pipe. Only as much liquid can flow from the pipe to the filtrate tank as is separated from the fiber mat to the pipe. The level of the liquid in the pipe is determined so as to cause some volatilization of the liquid in the pipe by the vacuum created by the liquid column. The vacuum is determined by the length of the efficient suction leg (drop leg), i.e. the unbroken column of the liquid and gas mixture, and by the density of the mixture.
Despite the fact that modern mills use more and more pressurized thickening devices, they still employ a drop leg which in an apparently advantageous way increases the pressure difference over the fiber layer. However, either a ten meters deep space must be provided for the drop leg itself and the filtrate tank from which space the filtrate is pumped up for further treatment, or the thickener or the washer must be arranged in the mill at the level of ten meters above the point where the stock to be treated must be pumped. Thus, the employment of a drop leg involves extra investments already at the planning and construction stage of a mill. Further, either the filtrate or the fiber suspension to be treated must be pumped up for the said ten meters which of course means unnecessary energy consumption.